1. Field of the Invention
The present invention relates to a diversity type transmitter used for mobile communication and particularly, to a diversity type transmitter having a delay time control system for controlling the delay time of each of plural transmission units so that the difference in delay time among the transmission units is within a permissible value range.
2. Description of the Related Art
In a transmission diversity system in which the same modulation waves are transmitted from plural transmission units at the same time and the modulation timing is made coincident among these modulation waves at a reception point to thereby achieve a diversity gain, it is required that the modulation timing at the transmission output terminal is within a permissible value range (coincident desirably). Accordingly, it is required that plural diversity type transmission units reduce the difference in delay timing of modulated data thereof so that the difference is within a permissible value range.
Dispersion in delay time among circuit parts is considered as a factor of inducing the difference in delay time among the transmission units. Particularly, the effect of the dispersion in delay amount among IF filters to be mounted for an intermediate frequency (IF) before it is converted to RF frequency is large. Further, the effect degree of the dispersion in delay time among these IF filters is increased as the transmission rate of the modulated data (chip rate) increases. Therefore, a CDMA (Code Division Multiple Access) type mobile communication system for carrying out high-speed data transmission or the like need the control of the delay time in each transmission unit in order to implement a diversity type transmitter.
FIG. 1 is a diagram showing a transmitter used a conventional delay time control system.
According to the transmitter shown in FIG. 1, signals generated by base band signal generator 101 are delayed by delay circuits 102a and 102b of transmission units 100a and 100b, passed through modulators 103a and 103b, frequency converters 104a and 104b and amplifiers 105a and 105b, and then finally output as RF (Radio Frequency) signals A and B. In order to control the delay amount of the signals, delay amount control circuit 108 controls the delay amount with delay amount set value 109 which is calculated in advance. The delay amount set value 109 is calculated by measuring the delay amounts of the transmission units 100a and 100b which are actually used.
In the transmitter described above, the delay amounts of the transmission units must be actually measured in order to achieve the delay amount set value 109, and thus the productive efficiency is low. The delay amount set value 109 is set only at the time when a diversity type transmitter is produced. Therefore, when the difference in delay amount is varied due to a temperature variation or a secular change under operation, an expected diversity gain may not be obtained.